Dangers of abrasive cleaning to historic buildings, undated

NYC 9/11 Public Portal Document

Preservation Brief 6: Dangers of Abrasive Cleaning to Historic Buildings Page 7 of 12

these statues undergo a lengthy process of polishing. Coatings are applied which protect the surface from corrosion, but they must be renewed every 3 to 5 years. A similarly delicate cleaning technique employing glass beads has been used in Europe to clean historic masonry structures without causing damage. But at this time the process has not been tested sufficiently in the Unite States to recommend it as a building conservation measure.

Sometimes a very fine smooth sand is used at a low pressure to clean or remove paint and corrosion from copper flashing and other metal building components. Restoration architects recently found that a mixture of crushed walnut shells and copper slag at a pressure of approximately 200 psi was the only way to remove corrosion successfully from a mid-19th century terne-coated iron roof. Metal cleaned in this manner must be painted immediately to prevent rapi recurrence of corrosion. It is thought that these methods "work harden" the surface by compressing the outer layer, and actually may be good for the surface of the metal. But the extremely complex nature and the time required by such processes make it very expensive and impractical for large-scale use at this time.

Cast and wrought iron architectural elements may be r, gently sandblasted or abrasively cleaned using a wire ;~ brush to remove layers of paint, rust and corrosion. Sandblasting was, in fact, developed originally as an , efficient maintenance procedure for engineering and industrial structures and heavy machinery--iron and steel bridges, machine tool frames, engine frames, and railroad rolling stock--in order to clean and prepare A them for repainting. Because iron is hard, its surface, i (ll which is naturally somewhat uneven, will not be ° damaged by controlled abrasion. Such Cast Iron may be abrasively cleaned, but must be treatment will, however, result in a small amount of painted immediately to prevent rust. Photo: NPS files. pitting. But this slight abrasion creates a good surface for paint, since the iron must he repainted immediately to prevent corrosion. Any abrasive cleaning of metal building components will also remove the caulking from joints and around other openings. Such areas must be recaulked quickly to prevent moisture from entering and rusting th metal, or causing deterioration of other building fabric inside the structure.

When is Abrasive Cleaning Permissible? For the most part, abrasive cleaning is destructive to historic building materials. A limited number of special cases have been explained when it may be appropriate, if supervised by a skilled conservator, to use a delicate abrasive technique on some historic building materials. The type of "wet grit" cleaning which involves a small amount of grit injected into a stream of low pressure water may be used on small areas of stone masonry (i.e., rough cut limestone, sandstone or unpolished granite), where milder cleaning methods have not been totally successful in removing harmful deposits of dirt and pollutants. Such areas may include stone window sills, the tops of cornices or column capitals, or other detailed areas of the facade.

This is still an abrasive technique, and without proper caution in handling, it can be just as harmful to the building surface as any other abrasive cleaning method. Thus, the decision to use this type of "wet grit" process should be made only after consultation with an Industrial Interiors that are not experienced building conservator. Remember that it is very time finely milled may be abrasively consuming and expensive to use any abrasive technique on a historic cleaned, in some instances. Photo: NPS flies. building in such a manner that it does not cause harm to the often fragile and friable building materials.

http://www2.cr.nps.gov/tps/briefs/brief06.htm 03/05/2002

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